1. Field of the Disclosure
The invention relates generally to the use of microwave energy to selectively heat thermoplastic polymer systems. The polymer systems may either be inherently responsive to microwave energy or modified by incorporating appropriate microwave responsive additives in the polymer or as components on the backbone of the polymer.
2. Background
Thermoplastic polymer pellets typically must be melted, re-shaped and cooled in a primary conversion process, such as extrusion or injection molding, in order to make parts of commercial value. In some instances, a secondary fabrication process, such as thermoforming, which involves further heating, reshaping, and cooling is required to achieve parts of commercial value. In both primary and secondary processes, heat energy is applied to the thermoplastic and is subsequently removed after reshaping has occurred.
Conventional heating mechanisms for thermoplastic polymer systems in many instances rely on contact or radiant heat sources. Radiant energy, commonly referred to as infrared, has a wavelength in the range of 1 to 10 microns and will penetrate absorbing materials to a depth of approximately 1 to 2 microns before half of the available energy has been dissipated as heat. The process of heat transfer continues through a process of conduction (in the case of a solid material) or a combination of conduction, convection and mechanical mixing in the case of a molten material. Contact heating similarly relies on conduction (or a combination of conduction, convection, and mixing) from the hot contact surface to heat the “bulk” of the material.
The rate of heat transfer (RHT) associated with a conductive heat transfer process can generally be described by the relationship: RHT=f(A, Ct, Delta T), where A is the area available for heat transfer, Ct is the thermal diffusivity of the material, and Delta T is the available temperature driving force, which will decrease with time as the temperature of the material being heated increases. The thermal diffusivity, Ct, of unmodified thermoplastics is inherently low, thereby impeding the heat transfer in a conventional radiant or contact heating system. Furthermore, the heat conduction process may result in an undesirable temperature gradient with the surface of the part being heated (such as a sheet material) being substantially hotter than the center of the part being heated, and being highly dependent on the thickness distribution of the part being heated.
By way of contrast, microwaves have a wavelength of approximately 12.2 cm, large in comparison to the wavelength of infrared. Microwaves can penetrate to a much greater depth, typically several centimeters, into absorbing materials, as compared to infrared or radiant energy, before the available energy is dissipated as heat. In microwave-absorbing materials, the microwave energy is used to heat the material “volumetrically” as a consequence of the penetration of the microwaves through the material. However, if a material is not a good microwave absorber, it is essentially “transparent” to microwave energy.
Some potential problems associated with microwave heating include uneven heating and thermal runaway. Uneven heating, often due to the uneven distribution of microwave energy through the part, may be overcome to a certain extent, such as in a conventional domestic microwave oven, by utilizing a rotating platform to support the item being heated. Thermal runaway may be attributed to the combination of uneven heating outlined above and the changing dielectric loss factor as a function of temperature.
Microwave energy has been used, for example, to dry planar structures such as wet fabrics. Water is microwave-sensitive and will evaporate if exposed to sufficient microwave energy for a sufficient period of time. However, the fabrics are typically transparent to microwaves, thereby resulting in the microwaves focusing on the water, which is essentially the only microwave-sensitive component in the material. Microwave energy has also been used to heat other materials, such as in the following references.
U.S. Pat. No. 5,519,196 discloses a polymer coating containing iron oxide, calcium carbonate, water, aluminum silicate, ethylene glycol, and mineral spirits, which is used as the inner layer in a food container. The coating layer can be heated by microwave energy, thereby causing the food in the container to brown or sear.
U.S. Pat. No. 5,070,223 discloses microwave-sensitive materials and their use as a heat reservoir in toys. The microwave-sensitive materials disclosed included ferrite and ferrite alloys, carbon, polyesters, aluminum, and metal salts. U.S. Pat. No. 5,338,611 discloses a strip of polymer containing carbon black used to bond thermoplastic substrates.
WO 2004048463A1 discloses polymeric compositions which can be rapidly heated under the influence of electromagnetic radiation, and related applications and processing methods.
A key limitation to the use of microwaves for heating polymeric materials is the low microwave receptivity of many useful polymers. The low microwave receptivity of the polymers thus requires either high powers or long irradiation times for heating such polymeric systems. In polymers designed specifically for microwave absorption, there is often a trade-off between their microwave properties and mechanical or thermal properties, i.e., the mechanical and thermal properties are often less than desirable.
Accordingly, there exists a need for processes and polymeric materials which facilitate the rapid, volumetric heating of the polymer using microwave energy. Additionally, there exists a need for processes and polymeric materials that have the ability to heat or melt only a portion of the polymeric material, sufficient to render the bulk material capable of flow, facilitating the shaping or further processing of the polymer.